[Eliot Ness]

I don't think you can get smaller than 1/4" unless you go to a fine wire upholstery stapler, and all that I've seen are air powered.  If you think 1/4" is too long put a small piece or two of cardboard between the backer board and the stapler.

I think John Goller uses a regular office stapler with the base removed.... that seems like a pretty thin staple to me, but it works for him.